Antecedentes: dinossauro pré-histórico que sobreviveu (assim como Nessie :))
Motivação: Mudou ao longo do tempo, de uma criatura que queria ser deixada sozinha, para um animal violento e cruel, para um salvador ocasional da Terra, para um animal em grande parte territorial.
Resposta completa:
Para ser honesto, muitas dessas perguntas foram bem explicadas (e referenciadas) na página da Wikipédia :
Within the context of the Japanese films, Godzilla's exact origins vary, but it is generally depicted as an enormous, violent, prehistoric sea monster awakened and empowered by nuclear radiation.
Assim, pode-se ver que, embora seja certamente fortalecido pela radiação nuclear (e, portanto, era muito atual após a sua liberação), não foi criado por isso. Na verdade, sua origem é muito anterior :
In the late Cretaceous era (70 million years ago) there was a species of therapod dinosaur resembling a large Tyrannosaurus, but with special amphibious adaptations which allowed it to swim between the islands upon which it hunted. This dinosaur species has been dubbed "Gojirasaurus". A minimal breeding population of gojirasaurs somehow survived the great extinction event which killed off other dinosaurs and continued into the modern era...
Esta descrição corresponde ao pano de fundo de Godzilla no filme de 2014, que você mencionou especificamente. Esta citação é da publicidade de um brinquedo oficial do filme :
Possibly the last of an ancient species of giant amphibious creatures that evolved when the surface of the Earth was over ten times more radioactive than it is today. GODZILLA can create his radiation stores into a violet, focused exahlation of atomic ray. Rarely seen, but spoken of in ancient island mythers, "Goijara" was last spotted in 1954, when the U.S. Navy encountered him and attempted to kill him with an atomic blast in the Pacific Ocean. Since then, the giant creature has been living in the deep ocean - until a theat to his survival from an ancient foe forces him to appear.
Quanto ao foco principal de suas perguntas - sua lealdade e motivações - eu acho que todos os filmes precisam ser examinados para ver como seu "personagem" se desenvolveu e mudou. Novamente no Wiki:
Godzilla's allegiance and motivations have changed from film to film to suit the needs of the story. Although Godzilla does not like humans, it will fight alongside humanity against common threats. However, it makes no special effort to protect human life or property and will turn against its human allies on a whim. It is not motivated to attack by predatory instinct: it doesn't eat people, and instead sustains itself on radiation and an omnivorous diet.
When inquired if Godzilla was "good or bad", producer Shogo Tomiyama likened it to a Shinto "God of Destruction" which lacks moral agency and cannot be held to human standards of good and evil. "He totally destroys everything and then there is a rebirth. Something new and fresh can begin."
O parágrafo acima (com referências removidas) mostra que Godzilla tende a não seguir nenhum mestre senão ele próprio, agindo como ele achar melhor. Isso é discutido em mais detalhes no Wiki específico do Godzilla :
Com tanta eficácia, Godzilla é perfeitamente capaz de lutar e se defender, e até de resgatar seres humanos, mas deseja em grande parte proteger a si mesmo e a seu território.Godzilla, in the original film...is an animal with semi-sapience that stumbles upon human civilization without any malicious intent, only destroying man-made structures or obstacles like buildings when the humans provoke him...In [later films] Godzilla is a malicious entity created from the restless souls of the dead from World War II. [Since then he has] developed as a character, and has since become a savior of the Earth, saving the world from other monsters like King Ghidorah, the Showa MechaGodzilla, etc.
According to Mothra's Shobijin's translation of Mothra, Rodan and Godzilla's conversation in Ghidorah, the Three-Headed Monster, Godzilla only "hates humans because they hate him."
In Legendary Pictures' Godzilla, Godzilla's behaviour seems to be that of a territorial animal.
Gareth Edwards, o diretor do filme, corroborou essa idéia em uma
His answer: the last samurai.
"He's an ancient warrior who's the last of his kind, and his kind has long since died out," Edwards says of his take on the legendary creature. "He lives a very solitary lonely existence and he's very happy to keep away from everyone, but we keep doing things to force him to return and put things right." ...
To see Godzilla's motivation and understand his reactions "when he was doing his thing," Edwards adds, "we dialed some more personality and made him a lot more human than we thought we would."
Então você pode ver que o objetivo do diretor era torná-lo territorial, mas essencialmente "humano" por natureza - um guardião que quer ficar sozinho, mas continua voltando para nos proteger (um tema bastante comum em muitos super-heróis filmes hoje em dia também.